1. Field of the Invention
The present invention relates to a boot which covers constant velocity universal joints, which are essential for joints for the drive shaft of front wheel drive vehicles, to inhibit water and dust from intruding into the joint part of constant velocity universal joints.
2. Description of the Related Art
The joint part of a constant velocity universal joint has been covered with a bellows-shaped boot, in which grease is sealed, conventionally. The boot inhibits water and dust from intruding into the joint part, thereby keeping the joint part rotating smoothly at large angles. Such a boot for a constant velocity universal joint comprises a major-diameter cylinder, a minor-diameter cylinder, and a substantially-truncated-cone-shaped bellows. The major-diameter cylinder is held to a joint outer race. The minor-diameter cylinder has a smaller diameter than that of the major-diameter cylinder, and is held to a shaft. The bellows connects the major-diameter cylinder with the minor-diameter cylinder integrally. In service, since the bellows deforms depending on the change of angles (i.e., joint angles) between the joint outer race and the shaft, it is possible to securely seal the joint part of the constant velocity universal joint with the boot even when the joint angles enlarge.
Such a boot for a constant velocity universal joint has been used to be formed of rubber. However, since rubber exhibits limitation in view of the durability, thermoplastic elastomer, which is good in terms of the weatherability, heat resistance and fatigue resistance, has been used recently. On the other hand, it is required to provide a boot for a constant velocity universal joint with a sealing function for inhibiting water and dust from intruding into the joint part reliably. However, the major-diameter cylinder of a boot has often been fitted around a mating member by means of irregularly-shaped fitting or noncircular fitting. Due to the circumstances, there has been a problem in that it is difficult to secure the sealing ability, because thermoplastic elastomer, the material for forming a boot, exhibits low elasticity so that it does not exhibit following ability as good as rubber. Moreover, blow molding is a convenient method for molding a boot for a constant velocity universal joint. However, since the surface of a mating member is formed as a noncircular shape generally, it is needed to mold a boot so as to conform to the noncircular shape. When manufacturing such a boot by blow molding, however, it is difficult to form the inner peripheral surface of the boot's major-diameter cylinder so as to enhance the sealing ability with respect to the mating member. Thus, from the manufacturing viewpoint as well, it is difficult to secure the sealing ability.
Hence, Japanese Unexamined Utility Model Publication (KOKAI) No. 2-87,131 discloses a boot for a constant velocity universal joint, boot which comprises a boot body formed of polyester-based thermoplastic elastomer, and an annular grommet made of soft rubber which is fitted into the boot body's major-diameter cylinder. The grommet can be manufactured with good accuracy by injection molding even when the grommet has thick-walled portions and thin-walled portions and has an inner peripheral surface formed as a noncircular shape, because the grommet can be molded independently of the boot body. Moreover, the boot body can be manufactured by blow molding, because it is not necessarily required to mold the boot body with such high configuration accuracy. In addition, a fastening force resulting from clamping is transmitted to the grommet by way of the boot body's major-diameter cylinder, and thereby the grommet deforms elastically to closely adhere to a mating member. As a result, the boot demonstrates the sealing ability. That is, the boot body can secure the durability of the boot, and the grommet can secure the sealing ability of the boot with respect to a mating member. Moreover, the boot can be manufactured inexpensively, because the boot body whose configuration is larger than that of the grommet can be manufactured by blow molding so that the man-hour requirement for manufacturing the boot is reduced.
Further, in recent years, since the performance of thermoplastic elastomer has improved, it is about to carry out manufacturing a boot for a constant velocity joint boot, boot which comprises a major-diameter cylinder with a grommet integrated, using an injection blow molding method, for instance, as proposed in Japanese Unexamined Patent Publication (KOKAI) No. 2003-276,944.
A boot for a constant velocity universal joint, boot which is provided with such irregularly-shaped cross-sectional portions on the inner peripheral side, is assembled with a mating member in the following manner: the grommet and major-diameter cylinder, or only the major-diameter cylinder is fitted around a mating member; and the major-diameter cylinder is fastened to the mating member by a clamp which is disposed around the outer periphery of the major-diameter cylinder. Therefore, when fitting the boot around the mating member, it is required to carry out positioning the boot for fitting the mating member into the boot. Accordingly, the positioning has been conventionally carried out in the following fashion: concavities are formed in the outer peripheral surface of the mating member, and at the same time positioning protrusions are formed correspondingly on the inner peripheral surface of the grommet or on the inner peripheral surface of the major-diameter cylinder; and the positioning protrusions are engaged with the concavities to position the boot with respect to the mating member.
However, since the major-diameter cylinder is relatively hard, the diametrically-enlarging deformation of the major-diameter cylinder might be restricted when the positioning protrusions slide on the outer peripheral surface of the mating member during the fitting of the boot around the mating member. Accordingly, there has been a problem in that the major-diameter cylinder might resist to the fitting of the boot around the mating member to degrade the operability. Moreover, even when the positioning protrusions engage with the concavities of the mating member during the fitting of the boot around the mating member, the deformation magnitude of the major-diameter cylinder is scant so that no fitting completion feeling might be obtained. Consequently, there has been a drawback that no checking might be made whether the boot is positioned with respect to the mating member.
Moreover, ring-shaped sealing protrusion rims are formed on the inner peripheral surface of the grommet or major-diameter cylinder, and are disposed within a fastening allowance of the clamp. Moreover, the positioning protrusions have often been provided with a sealing protrusion rim at the leading end. In this instance, however, since the sealing protrusion rims slide on the mating member when fitting the boot around the mating member, the sealing protrusion rims might be worn off or damaged. If such is the case, the boot might suffer from a drawback in view of the sealing ability.
Hence, Japanese Unexamined Utility Model Publication (KOKAI) No. 2-71,122 discloses a boot for a constant velocity universal joint, boot which comprises slits extending axially from an opposite end surface of a major-diameter cylinder, and positioning protrusions formed so as to avoid the slits. With such an arrangement, since the slits make it easy to diametrically enlarge the major-diameter cylinder, it is possible to reduce the resistance during the fitting of the boot around a mating member. However, it is less likely to obtain a fitting completion feeling when the positioning protrusions engage with the mating member's concavities. Moreover, when considering employing such a construction for a boot for a constant velocity universal joint, boot which comprises a boot body and a grommet, it is not recommendable to form the slits in the grommet or major-diameter cylinder from the viewpoint of the sealing ability, in addition to the problem of the poor fitting completion feeling. Besides, there arises a problem in that the man-hour requirements are enormous for manufacturing such a boot.